Pulse-echo ultrasonic imaging technology has become a vital tool for clinicians for examining the internal structure of living organisms. In the diagnosis of various medical conditions, it is often useful to examine soft tissues within the body to show structural details of organs and blood flow in these organs. Experienced clinicians can use this information in diagnosing various pathologies.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties of the echoes (e.g., amplitude and phase) from targets at varying depths within the body. Typically, the ultrasound beam is focused at various depths within the body in order to improve resolution or image quality. The echoes are received by a transducer, typically the same transducer used for transmission, and processed to generate an image of the object, usually referred to as a B-scan image.
The transducers used in ultrasonic imaging are typically multiple element arrays in which electronic processing is used to focus the ultrasonic beam to improve lateral resolution. To focus the beam, the acoustic energy radiating from the transducer is concentrated or aimed at a single point, called the focal point. The ultrasound waves launched by each element of the array are time delayed with respect to each other, such that the waves from each element add up constructively at the focal point, creating a large acoustic intensity. In earlier ultrasound imaging systems which used a single element transducer, this focusing was usually performed by an acoustic lens or by curving the element itself. The focusing method used in an array, thus, simulates the focusing of the lens.
Upon reception of the ultrasound echoes from the body, each element is connected to a receiving circuit containing time delay circuits which are used to focus the transducer during reception. After each transmission pulse, the transducer waits for the receiving circuit to receive the echoes from the furthest depths before it sends out another transmission pulse.
For multi-element array transducers, if the same element time delays are utilized in transmission and reception, then the transmission and reception beam patterns will be identical. However, differences exist between transmit and receive focus. To focus ultrasonic waves on transmit, the time delays are fixed, such that the beam focal parameters are irreversibly set for that particular transmission pulse. Therefore, with transmit focusing, the focus cannot be changed, once the pulse has been launched.
On reception, however, the time delays can be changed continually and rapidly so as to follow the pulse as it propagates into the body at the speed of sound, thus forming a time-varying and focused received beam pattern. Thus, the received ultrasound beam can be kept in focus over a wide range of depths. By shifting the receiver focus in this manner, the best spatial resolution and image quality is obtained.
Since the transmitted beam can only be focused from pulse to pulse, it is common to create an image focused at a specific and operator selectable depth. This is called single zone focusing.
An extension of single zone focusing uses multiple sets of focused transmitted beams to create a composite image. Each set is focused at a specific depth. This is usually called multiple zone focusing. Generally, the number of zones varies from three to five zones. In this manner, real-time images are obtained with improved focus in certain regions in the image. This increase in image quality usually results in lower frame rates. Lower frame rates mean that moving structures or blood flow are not easily imaged and diagnosis may be impaired.
Frame rate reduction typically occurs with multi-zone focusing because each transmit zone requires almost the same acquisition time as a single zone, even though only a portion of the received data is used in the final display. The reason for this seemingly excessive time is because significant received signals continue to be received from regions in the image beyond the focal depth. Thus, a new transmit pulse for focusing at a zone cannot be sent until the entire line has been received. Otherwise, data from one zone would interfere with the data from another zone. The time delays between transmission of the separate pulses for an entire line of the image are referred to as the critical time delays. In practice, due to low frames rates, only a few zones are ever used.
The prior art has attempted to alleviate this disadvantage of low frame rates by such expedients as increasing the intensity of pulses transmitted to deeper zones compared to pulses transmitted to nearer zones, but such techniques have not resulted in any significant improvement in frame rates or in the number of zones available at a given frame rate, as a practical matter.
As will be seen, the present invention provides methods for increasing the number of zones without decreasing the frame rate together with a comprehensive method for optimizing these parameters. These methods permit a large number of zones to be used in practice. In effect, this results in simultaneous transmit and receive focusing at all depths in the image.